Don't crash on 'decltype(auto)::'. Rather than treating it as a meaningless

nested-name-specifier (as the standard appears to require), treat it as the
type specifier 'decltype(auto)' followed by a nested-name-specifier starting
with '::'.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294506 91177308-0d34-0410-b5e6-96231b3b80d8